The present invention relates to multi-axis internal spinal fixation. In more detail, the present invention relates to an internal spinal fixation system, and a method of stabilizing, or fixing, the spine for use with either bilateral rods or plates such as the Steffee/variable screw placement system or a central rod and plurality of cross-bars or plates such as the so-called Tacoma Monorail System, utilizing wedge-shaped and/or flat washers having off-set and/or centered openings therein to provide multiple axes for the pedicle screws used to fix the rods, cross-bars, and/or plates to the vertebrae of the patient.
There are many systems available for internal fixation of the spine. Such systems are described in the patent literature (see, for instance, U.S. Pat. Nos. 4,696,290, 5,092,866, and 5,129,899) and the scientific literature (see, for instance, D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine (Philadelphia: Nanley and Belfus, Inc.) 1992 and H. S. An and J. M. Cotler (Eds.), Spinal Instrumentation (Baltimore: Williams and Wilkins) 1992), and are available from such vendors as AcroMed, Smith and Nephew, MOSS(copyright) Miami, Osteonics, Sofamor Danek, and others.
A problem with all such systems, however, is the joint between the screws used to affix the system to the pedicle and the rods, cross-bars, and/or plates of the system. As stated in J. M. Cotler, et al., Principles, Indications, and Complications of Spinal Instrumentation: A Summary Chapter, in H. S. An and J. M. Cotler, Spinal Instrumentation pp. 435-456 (Baltimore: Williams and Wilkins) 1992, xe2x80x9c[a] significant problem in pedicular screw fixation appears to be at the site of linkage between the screw and rod or plate.xe2x80x9d
It appears that the problems at the site of this linkage may result from the geometry of the joint between the screw and the rod or plate. This difficult geometry results from several factors, including the different angles and placement of the vertebrae and their relative sizes, the shape of the vertebrae and the spacing between vertebrae, the placement of the screws, the lordosis of the spine, and the need to insert the screws into each vertebra at an angle. With regard to the angle of the pedicle screws, pedicle screws are angled inwardly and upwardly into the vertebra for maximum strength and, because the surfaces of the pedicles of each vertebrae are angled relative to each other, the screws rarely line up across the vertebral body into which they are screwed. Nor do they line up from one vertebra to the adjacent vertebra even if the adjacent vertebrae are the same size and shape (which they generally are not). For a more complete discussion of the biomechanics of the bone-implant interface, reference is made to H. A. Pool and R. W. Gaines, Biomechanics of Transpedicular Screw Spinal Implant Systems, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 37-44 (Philadelphia: Nanley and Belfus, Inc.) 1992, M. R. Pinto, Complication of Pedicle Screw Fixation, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 45-54 (Philadelphia: Nanley and Belfus, Inc.) 1992, and M. H. Krag, Vermont Spinal Fixator, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 121-145 (Philadelphia: Nanley and Belfus, Inc.) 1992, which references are incorporated herein in their entirety by these specific references thereto. Because the pedicle screws do not line up, the rod which runs along the longitudinal axis of the patient""s spinal column, which provides the structural rigidity required to stabilize the spine, must either be bent to the location of each screw head or the stabilizer must be provided with adjustable structure which enables the screw head to be attached to the rod.
As a result of this difficulty, the literature includes comments such as the following statement in R. M. Puno and J. A. Byrd III, Transpedicular Screw/Rod Fixation Using the Puno-Winter-Byrd(PWB) System, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 83-106 (Philadelphia: Nanley and Belfus, Inc.) 1992:
xe2x80x9cTranspedicular fixation has been proved to be of value in the treatment of spinal disorders . . . However, experience has shown that this method of instrumentation places great demand on the surgeon""s skill because of the anatomic constraints related mainly to the anatomy and morphometry of the spinal pedicle.xe2x80x9d
Many of the above-listed systems, and many of the systems described in the literature, attempt to relieve this burden on the surgeon by providing angled screws (for instance, the AMSET(copyright) R-F reduction-fixation system), so-called polyaxial screws (for example, the MOSS(copyright) Miami system noted above), full-length, scalloped, open-slot plate design with an undersurface complementary to the shape of the screw head (the Sofamor Danek plate and screw system noted above for example) for optimal positioning of the screws and up to 15xc2x0 medial-lateral and 30xc2x0 craniocaudal angulation at the screw-plate interface, and infinitely variable couplers (the so-called Rogozinski spinal rod system for example) which are said to allow rotation through a 130xc2x0 arc to allow screw placement within the pedicle with no requirement to align each screw with the screw in the adjacent vertebrae.
Although these prior systems address these problems, as evidenced by the fact that new systems are introduced by the same vendors which are already marketing the above-listed systems, no currently available system completely solves all the problems presented by the need for optimal screw placement, angulation of the screw, and effective load transfer from spinal column to implant. An ideal system would (a) accomodate optimal screw placement, height, and angulation, (b) accomodate different sizes and shapes of vertebrae, (c) minimize (or not require) bending or other fabrication during surgery, (d) maintain an angle of approximately 90xc2x0 between the screw head and the plate or cross-bar to which the screw is attached for effective load transfer from spinal column to implant and to minimize the likelihood of slippage and/or gross failure, and (e) be strong enough to provide lasting and rigid fixation of the spine. Those skilled in the art will recognize that this list is not exhaustive, but is instead intended to illustrate some of the desirable characteristics of an ideal internal fixation system. Other design criteria are also important, and some practicioners may consider some criteria so important that they might not even list others.
So far as is known, none of the above-listed internal fixation systems meets these criteria in every patient. The disadvantages and limitations of currently available systems are made clear from reports in the literature of failure rates (failure of the device, not such complications as infection, phlebitis, seroma, neurologic deficit, etc.) as high as 25% (see R. Roy-Camille, et al., 203 Clin. Orthop. 7 (1986)), 11% (see, S. F. Heim and E. R. Luque, Danek Plate and Screw System, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 201-234 (Philadelphia: Nanley and Belfus, Inc.) 1992), 8% (see, R. M. Puno and J. A. Byrd III, Transpedicular Screw/Rod Fixation Using the Puno/Winter/Byrd (PWB) System, supra), and 2-7% D. M. Arnold and L. L. Wiltse, The Wiltse System of Internal Fixation for the Lumbar Spine, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 55-82 (Philadelphia: Nanley and Belfus, Inc.) 1992.
The currently available systems have other limitations. By way of example, so far as is known, no currently available surgically implanted system can predictably treat rotoscoliosis. Further, no currently available system is conveniently used in multiple level surgery. Multiple level surgery is a challenge for the surgeon because of the need to align the pedicle screws in multiple vertebrae while working under the heavy muscles of the back.
There is therefore a need for improvement of such systems, and it is this improvement to which the present invention is directed. In particular, it is an object of the present invention to improve the screw-plate interface in those systems in which the screw is angled and/or spaced at varying intervals. Another object of the present invention is to provide flexibility of placement, angulation, spacing, and screw height for accomodating the pedicle screws of such systems. Another object of the present invention is to provide a system which is universal in the sense that, although comprised of relatively few parts, it will work with pedicle screws and laminar hooks, thereby providing even more flexibility and ease of use. Another object of the present invention is to provide an internal spinal fixation system which avoids surgery under the heavy muscles of the back so that the surgery is simplified and there is more room for fusion of adjacent vertebrae in the lateral gutter. Other objects, and the advantages, of the present invention will be made clear to those skilled in the art by the following description of the preferred embodiments thereof.
These objects, and other objects to be made clear by the following detailed description of the preferred embodiments of the invention, are met by providing a spinal stabilizer comprising a rod, a screw, a cross-bar having a hole therein, means for attaching the cross-bar to the rod, and a washer. The washer comprises a cylindrical body having one end angled with respect to the side walls of the cylindrical body and a longitudinal passage therethrough for receiving the screw for affixing the cross-bar to the vertebra of a patient. The washer also comprises means for resting on and rotatably engaging the margins of the hole in the cross-bar so that the body is capable of being rotated in the hole in the cross-bar to provide an infinite variety of angles and pedicle screw placements while maintaining an optimal interface between the head of the screw and the washer so as to effectively transfer the load from the spinal column to the cross-bar.
In another aspect, the present invention contemplates a novel washer for use in connection with an internal spinal stabilizer which comprises a wedge-shaped, cylindrical body with a hole through the body which is offset from the center of the longitudinal axis of the washer. The hole receives a screw for affixing an internal spinal stabilizer to the vertebral body of a patient. Means is formed on said body for rotatably engaging the spinal stabilizer to allow the body to rotate around the 360xc2x0 of the hole to provide infinite variability in the angle and location of the interface between the screw and the plate.
The present invention also provides a method of affixing a spinal stabilizer to the vertebra of a patient, the stabilizer comprising a washer having an off-center passage therethrough, a cross-bar, and a screw, comprising the steps of engaging the cross-bar with the washer, driving the screw into the vertebral body through the passage in the washer, and changing the angle of the screw relative to the vertebral body to which the cross-bar is affixed by rotating the washer relative to the cross-bar.
In a second embodiment of the method of affixing a spinal stabilizer to the vertebra of a patient, the spinal stabilizer comprises a washer having a cylindrical body with one end angled with respect to the side walls of the cylinder and a longitudinal passage therethrough, a cross-bar, and a screw, and the method comprises the steps of engaging the cross-bar with the washer, driving the screw into the vertebral body through the passage in the washer, and changing the angle of the screw relative to the vertebral body to which the cross-bar is affixed by rotating the washer relative to the cross-bar.